Time-Domain Transmission Sensor System for On-site Dielectric Permittivity Measurements in Soil: A Compact, Low-cost and Stand-alone Solution

Dielectric-based measurement techniques have been shown to be very effective in determining the properties of various materials. These techniques have been widely used in a variety of fields and applications. Time Domain Transmission (TDT) techniques have grown in popularity because they are practical, non-destructive, provide measurements in real time and produce accurate measurements that are independent of multiple reflections. TDT techniques, on the other hand, are mostly performed with specialized, bulky laboratory equipment, such as a Vector Network Analyzer (VNA) which makes TDT measurements prohibitively costly and unpractical. In fact, few works in the literature have reported portable on-site TDT systems. The aim of this paper is to design and implement a dedicated, compact, and low-cost microwave Time Domain Transmission (TDT) sensor for measuring superficial soil dielectric properties on-site. Our sensor uses a time-delay measurement technique over a microstrip transmission line to estimate the dielectric properties of the soil under test. Measurement results show that the computed mean absolute error (MAE) is less than 1.2 when compared to a calibrated dielectric assessment kit (DAK) with soils containing less than 20 % of water (εr<5.0), implying that our TDT sensor system can obtain on-site measurements in relatively dry soils with acceptable accuracy.基于介电特性的测量技术（Dielectric-based measurement techniques）已被证实可高效测定各类材料的性能，该类技术已在诸多领域与应用场景中得到广泛应用。时域透射（Time Domain Transmission, TDT）技术因具备实用性、非破坏性、实时测量能力，且可输出不受多重反射干扰的精准测量结果，其应用普及率持续攀升。但此类技术目前大多需依托矢量网络分析仪（Vector Network Analyzer, VNA）等专业且体积庞大的实验室设备开展测量，这使得TDT测量的成本过高，难以实现实用化部署。事实上，现有学术文献中鲜有关于便携式现场TDT系统的研究报道。本文的研究目标为设计并实现一款专用、紧凑且低成本的微波时域透射（TDT）传感器，用于现场测定表层土壤的介电特性。本传感器基于微带传输线上的时延测量技术，对被测土壤的介电特性进行估算。测量结果表明，当土壤含水率低于20%（相对介电常数ε_r<5.0）时，与经过校准的介电评估套件（dielectric assessment kit, DAK）相比，本系统的计算平均绝对误差（mean absolute error, MAE）小于1.2，这意味着所提出的TDT传感器系统可在相对干燥的土壤中实现精度符合要求的现场测量。